Liquid developers for electrostatic images



us. Cl. 252--62.1

United States Patent 3,551,337 LIQUID DEVELOPERS FOR ELECTROSTATIC IMAGES Gene H. Robinson, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Sept. 1, 1967, Ser. No. 664,918

- Int. Cl. G03g 9/04, 13/00 8' Claims ABSTRACT OF THE DISCLOSURE .the finely ground pigments into an electrically insulating carrier liquid such as cyclohexane. Of the pigments used,

at least one is a phthalocyanine and serves as a charge control agent.

This invention relates to new liquid developers for developing electrostatic latent images and processes for preparing such developers.

With the present day technological expansion accompanied by an even greater growth in sheer volume of literature, technical and otherwise, the importance of information storage has taken on greater significance. One

method of storing information is an electrophotographic method utilizing what is termed a microimage. 'Microimages, as the name implies, are images that have been optically reduced in size as compared to the original. In electrophotography microimages are generally formed on a photoconductive element comprising a transparent film support bearing an optically clear, homogeneous film of a photoconductor. These microimages are normally used as intermediates for preparing further copies.

One step in the production of such copies is the development of the electrostatic latent image. To date both dry and liquid methods of development have been used for the development of microimages. However, in the case of dry toners, the minimum particle size to which the toner can be reduced is of the order of 2' to 5 microns. This .places a practical limit on resolution in the recorded image of "from 75 to 100* line pairs per millimeter. For a quality microimage such resolution is marignal at most. Normally, the acceptable minimum resolution is about 120 line =pairs per millimeter. In addition to the marginal resolu- Jtion one must also consider the reduction in resolution resulting from the heat or solvent vapor treatment generally'employed to permanently adhere such toners.

Many of the disadvantages of dry developers, such as dust, poor resolution, and external fixing means etc. can be avoided by the use of a liquid developer. Such developers'as disclosed initially by Metcalfe in J. Sci. Instr. 32: 74-75, February 1955, comprise a dispersion of suitably colored marking particles in an insulating liquid. Liquid developers are commonly made by milling a suitable pigment or dye with a binder in a liquid medium which is of "-sufiiciently resistivity that it will not disturb the charge image on the member to be developed and yet has sufficient solvent action on the binder to cause slight tackiness.

A serious drawback of prior art developers is that the ,particles, if reduced to their ultimate size, frequently tend to agglomerate or they clump together, unless care is taken in their preparation to avoid such. Agglomeration in liquid developers produces a serious loss in maximum resolving power as well as reduction in shelf life due to settling out of the agglomerated particles ln addition,

3,551,337 Patented Dec. 29, 1970 previous liquid developers were incapable of maintaining the charge polarity if the discrete particles upon storage in the final dispersed form. The cause of this serious disadvantage appears to be the result of attempting to control the charge by use of certain resins which are themselves often partially soluble in the carrier liquid used.

In addition, many previous attempts have been made to produce a black colored, stable, negative polarity, high resolution liquid developer. Such attempts have generally either failed or at most have resulted only in a very unsatisfactory product.

It is, therefore, an object of this invention to provide new liquid developers for electrostatic images, which developers are stable in the final dispersed form and have improved shelf life with little or no agglomeration.

It is another object of this invention to provide finegrained liquid xerographic developers that are capable of producing images of high resolution.

It is still a further object of this invention to provide a liquid Xerographic developer in which the charge polarity of the discrete particles is stable and not subject to change during storage in the final dispersed form.

It is still another object of this. invention to provide a novel black or dark colored, stable, negative polarity liquid xerographic developer.

These and other objects of this invention are accomplished by preparing a developer concentrate comprising an intimate mixture of at least two pigments and a resinous binder such as an alkyd resin. The concentrate can be prepared by conventional high shear milling means, such as a three-roller paint mill and the like. The concentrate thus formed can be kept for substantial periods and is quickly prepared for use by dispersing a small quantity in an electrically insulating carrier liquid, such as alkanes and cycloalkanes, for example, cyclohexane, n-decane, etc. If desired, a small quantity of an aryl or substituted aryl dispersing aid may be used as is described by York US. Pat. No. 3,135,695.

In order to obtain the fine particles needed to give a developer capable of forming images of high resolution it is necessary to use a milling or grinding technique which subjects the particles to very high shear forces. Normally such fine grinding is facilitated by using a viscous binder which does not influence the charge polarity of the particles. Useful binders include modified or unmodified alkyd resins, such as maleic alkyls, pentaerythitol modified alkyds, styrene alkyds, glyceryl phthalate linseed modified alkyds, linseed modified alkyds, etc. Among preferred binders are the alkyd resin lithographic ink type containing no diluents and which are viscous at room temperature. Good results are obtained with mixtures having a weight ratio of total pigment to binder of 1 to 9 with a useful ratio ranging from about 1:36 to about 1:1.

In the present invention the resinous binder, which can be partially soluble in the carrier liquid, is not used as a charge control agent. In accordance with this invention, the highly stable electrostatic charge of phthalocyanine pigments is utilized so as to control the charge polarity of the final liquid developer. Among the many useful phthalocyanine pigments are included phthalocyanine green (Solfast Green), phthalocyanine blue (Monastral Blue), metal-free phthalocyanine, etc.

In addition to the charge control agent, a second pigment, which preferably is considerably smaller in particle size than the control agent, is added to the developer concentrate. In this combination the charge control agent acts to stabilize the final dispersed product. For example, some carbon lampblacks alone give a toner particle having a weak positive charge when milled in various alkydtype resin binders. Formulation of the developer by mixing the carbon with phthalocyanine blue prior to milling, stabilizes charge to give a developer having a strong positive charge. Similarly, channel blacks give a toner particle which has a weak negative charge when incorporated in an alkyd or modified alkyd resin binder. Addition of phthalocyanine green to the carbon before milling stabilizes the negative charge to give a more satisfactory negative particle developed. Thus, the resultant liquid developer contains a mixture of at least two pigments, with at least one acting as a charge control agent and at least one other acting to give color, body, etc. The second pigment is preferably smaller in size than the control agent so that the second pigment in effect covers the control agent particles. However, in order to maximize charge stability, the polarity of the electrostatic charge of both pigments must be the same. A suitable weight ratio of charge control agent (or first type of pigment) to second type of pigment is 1 to 3 with satisfactory results being obtained with ratios of from about 1:7 to about 1:1.

The carrier liquid, into which the developer concentrate is dispersed, functions as a liquid insulating medium. Preferably, this liquid has a very high resistance and a low dielectric constant. In general, useful carrier liquids should have a dielectric constant of less than 3 and should be stable under a variety of conditions. Liquids of this nature are now well known in the art and would include cyclohexane, n-decane, etc. In the present invention, the developer concentrate or paste can be dispersed into, for example, 100 percent cyclohexane. However, in order to avoid problems of flammability, the present developers are often prepared by adding a non-flammable organic liquid as a flammability control agent which also serves as a diluent. Suitable liquids for this purpose would include halogenated hydrocarbon sovlents, for example chlorinated fiuorinated lower alkanes, such as trichloromonofluoromethane, trichlorotrifluoroethane, etc. A useful combination of flammability control agent to carrier liquid is 8 parts by volume of control agent to 2 parts by volume of carrier liquid. Satisfactory results are obtained with flammability control agent to carrier liquid volumetric ratios of from about 9:1 to about 3 :2.

The properties of the present developers can often be enhanced by the addition of a rosin, including hydrogenated rosin and esters of hydrogenated rosin. The rosin type additives improve the dispersability of the pigment concentrate in the liquid carrier. These additives also increase shelf life and improve resolving power. The rosin materials can be added in amounts of about 0.02 to about 2.0 weight percent based on the total weight of pigment, with especially good results being obtained using amounts of from about 0.1 to about 1.0 weight percent.

The present invention is further illustrated by the following examples which include preferred embodiments thereof.

EXAMPLE 1 Forty grams of phthalocyanine green pigment (Solfast Green, Sherwin-Williams Company, Cleveland, Ohio), is stirred into 160 grams of a glyceryl phthalate linseed modified alkyd vehicle (Dyal XLG-l07, Sherwin-Williams Company), until all of the pigment is completely wetted. The mixture obtained is placed in a three-roller paint mill with the rollers set for minimum gap (maximum shear). The paste is then collected from the third roller by means of a collector trough having a razor edge which contacts the third roller tangentially. The paste is then transferred by hand to the first roller again, for a second pass through the mill, and is subsequently passed through the mill a third time. One-tenth gram of this paste is dispersed with agitation into ml. of cyclohexane. The dispersion is then diluted to working strength by adding 80 m1. of 1,1,Z-trichloro-1,2,2-trifluoroethane to form a diluted developer. Next a latent positive charge image is produced on an electrophotographic element comprising a transparent poly(ethyleneterephthalate) support bearing a conductive layer upon which is coated a layer containing triphenylamine bound in a copolyester such as a poly(ethyleneterephthalate) made by replacing a part of the ethylene glycol normally used with a glycol derived from a bisphenol such as 2,2-bis[4-(;3-hydroxyethoxy) phenyl] propane and containing 2,6-bis(4-ethylphenyl) -4- (4-n-amyloxyphenyl thiapyrylium perchlorate as a sensitizer. The latent image is produced by charging the element positively to a surface potential of about 600 volts with the conductive layer grounded and exposing by contact with a resolving power test transparency bearing a pattern of dark lines on a clear background. The charged and exposed element is then developed by dipping it in the diluted developer prepared as described above. A positive reproduction of the original having a resolving power of in excess of 200 line pairs per millimeter results. The developed image is smudgeproof on drying of the carrier liquid.

EXAMPLE 2 Five grams of the phthalocyanine green pigment used in Example 1 is stirred into 180 grams of the resin of Example 1 until completely wetted. Next, 15 grams of a colloidal channel carbon black (Carbolac 2, manufactured by Cabot Corporation, Boston, Mass.), is stirred with care into the mixture. When all of the pigment has been wetted by the resin, the blend is transferred to a threeroller mill to complete dispersion of the pigments into the resin. Milling of the paste and preparation of a working strength developer are carried out as in Example 1. Next the electrophotographic element of Example 1 is charged negatively to a surface potential of about 600 volts and contact exposed to a resolving power test object bearing a pattern of clear lines on an opaque background. The electrostatic latent image is developed by dipping it briefly into the diluted developer prepared as described above. A pattern of dark lines on a clear background is obtained and has a resolving power of 250 line pairs per millimeter. The image is substantially smudgeproof upon evaporation of the carrier liquid.

EXAMPLE 3 Five grams of the phthalocyanine green pigment of Example 1 is stirred into 160 grams of a glycerol phthalate lineed modified alkyd vehicle (Beckolin 1451, sold by Reichold Chemicals, Incorporated, White Plains, N.Y.), until completely wetted. Next, 15 grams of the carbon used in Example 2 is stirred into the mixture until the carbon is all wetted. The mixture is milled as above in a three-roller mill for three complete passes through the mill. The milled paste is collected and 0.1 gram of paste is dispersed with agitation into 20 ml. of cyclohexane. After dispersion, the developer is diluted to working strength with ml. of 1,1,2-trichloro-l,2,2-trifluoroethane. Then the electrophotographic element of Example 1 is charged positively to a surface potential of about 600 volts and contact exposed to a resolving power test object of the type described in Example 1. The electrostatic latent image is developed by briefly dipping it into the developer. -A pattern of dark lines on a clear background is obtained, in which the resolving power is in excess of 200 line pairs per millimeter. The image is smudgeproof upon evaporation of the cyclohexane.

EXAMPLE 4 Five grams of phthalocyanine blue pigment (B-473l, sold by Allied Chemical Corporation), is stirred into grams of a pentaerythritol modified alkyd resin (Dyal XLP-104, manufactured by Sherwin-Williams Company), as in Example 1. Then 15 grams of carbon lampblack is stirred into the mixture until all wetted. The mixture is milled as in Examples 1, 2 and 3 and collected. One gram of the paste is then dispersed with agitation into 20 ml. of cyclohexane. More cyclohexane is then added to make a total volume of 500 ml. To this is added 500 ml. of l,l,Z-trichloro-l,2,2-trifluoroethane, to make the working strength developer. Now the electrophotographic element of Example lischarged negatively to a surface potential of about 600 volts and exposed by contact to a photographic negative bearing a pattern of substantially transparent lines on a high densitybackground. The element is then placed facing a plane conducting electrode at a distance of 0.020 inch therefrom, while the developer liquid is caused to flow between the image-bearing surface and the conducting electrode. A good, smudgeproof duplicate of the original is obtained on the surface of the electrophotographic element upon drying the carrier liquid.

EXAMPLE Forty grams of the phthalocyanine blue pigment of Example 4 is stirred into 160 grams of a pentaerythritol modified alkyd resin (Dyal XLP-307, manufactured by Sherwin-Williams Company), until wetted completely. The mixture is milled on a three-roller mill as described in the preceding examples and collected. One-tenth gram of the resultant paste is dispersed in ml. of cyclohexane with agitation. The dispersion so produced is diluted to working strength by carefully adding 90 ml. of 1,1,2-trichloro-1,2,2-trifluorethane. The electrophotographic element of Example 1 is charged positively to a surface potential of about 600 volts and exposed by contact to a photographic negative of the type described in Example 4. The latent image is developed by briefly dipping the element into the developer solution and withdrawing it. A positive reproduction of the negative results. The positive image of blue lines on a clear background is substantially smudgeproof upon evaporation of the carrier liquid.

EXAMPLE 6 A paste concentrate is made as in Example 2, substituting for the glyceryl phthalate linseed modified alkyd vehicle a linseed modified phthalic alkyd resin vehicle (Duraplex C-45-LV, sold by Rohm and Haas Company). The phthalocyanine green and carbon of Example 2 are used in the same proportion as previously with milling and collecting of the paste also as in Example 2. The final dispersion is made up by the procedure of Example 1. The electrophotographic element of Example 1 is then charged negatively to a surface potential of about 600 volts and contact exposed to a transparency bearing a pattern of opaque lines on a clear background. The element is placed facing a plane conducting electrode in the manner of Example 4. The conducting layer of the photoconductive element is held at a potential of 300 volts positive bias with respect to the facing conductive electrode whilethe developer solution is caused to flow in the gap between the photoconductive element and the facing electrode. The electrode is held in place with the bias turned on until all of the developer has drained away. The electrode is then removed after removing the bias potential. A smudgeproof negative-appearing image containing clear lines on a black background corresponding to the original image is produced on the photoconductive element.

EXAMPLE 7 A paste concentrate is made as in Example 2, using the same quantities of phthalocyanine green and Carbolac 2 carbon. After one pass of the paste through the threeroller mill, 0.02 gram of rosin (Water-White rosin, distributed by Scobell Chemical Company, Incorporated, Rochester, N.Y.), is added to the paste on the mill. The mixture is then passed through the mill two additional times and collected. Next a final dispersion is made up by the procedure of Example 1. The electrophotographic element of Example 1 is charged and exposed as before and developed by dipping it briefly into the developer dispersion. A positive reproduction of the test object results, which is smudgeproof upon drying of the carrier liquid, and which has a resolution of in excess of 400 line pairs per millimeter.

V 6 EXAMPLE 8 The procedure of Example 7 is again followed in making a paste concentrate. After one pass of the paste through the three-roller mill, 0.02 gram of a hydrogenated wood rosin (Staybelite, sold by Hercules Powder Company, Wilmington Del.), is added to the paste on the mill. The milling is completed as in Example 7, the paste collected and a working strength developer prepared as in Example 1. Charging, exposure and development are carried out as in Example 2. A positive-appearing image appears, which is smudgeproof upon drying of the carrier liquid and which has a resolution of in exces of 400 line pairs per millimeter.

EXAMPLE 9 The procedure of Example 7 is repeated using in place of the resin an ester of a hydrogenated wood rosin (Staybelite Ester I-C, Hercules Powder Company). Charging, exposure and development are carried out as in Example 6. A negative-appearing image results which is smudgeproof after drying of the carrier liquid and which has a resolution of in excess of 400 line pairs per millimeter.

The invention has been described in considerable detail with particular reference to certain preferred embodiments thereof; however, it will be understood that many variations and modifications can be effected within the spirit and scope of the invention as described above and as defined in the appended claims.

I claim:

1. A liquid developer for electrostatic images comprising an electrically insulating carrier liquid having dispersed therein a mixture of an alkyd resin binder which is partially soluble in the carrier liquid, a charge control agent comprising a phthalocyanine pigment, a colorant comprising a pigment dilferent from said phthalocyanine pigment and from about 0.1 to about 1% by Weight based on the combined weight of charge control agent and colorant of a dispersing aid selected from the group consisting of rosin, hydrogenated rosin and esters of hydrogenated rosin, said charge control agent and colorant having electrostatic charge of the same polarity and being present in a weight ratio of control agent to colorant of from about 1:7 to about 1:3.

2. A developer composition as in claim 14 wherein the weight ratio of charge control agent and colorant to resin is from about 1:36 to about 1:1.

3. A developer as in claim 1 wherein the carrier liquid is cyclohexane and the charge control agent is chosen from the group consisting of phthalocyanine blue and phthalocyanine green.

4. A developer as in claim. 1 containing a non-flamable organic liquid as a flammability control agent in a volumetric ratio of said agent to carrier liquid of from about 9:1 to about 3:2.

5. A developer as in claim 4 wherein the flammability agent is a halogenated lower alkane.

6. A negative polarity, black colored, liquid developer for electrostatic images consisting essentially of an electrically insulating carrier liquid comprising cyclohexane having dispersed therein an alkyd resin binder, a charge control agent consisting essentially of a chlorinated phthalocyanine pigment, a colorant comprising a carbon pigment having an eletcrostatic charge of the same polarity as the charge control agent, the weight ratio of said colorant to said control agent being from about 3:1 to about 7:1 and the weight ratio of said resin binder to the total weight of control agent and colorant being fromv about 1:1 to about 36:1, and a dispersing aid present in the amount of from about 0.1 to about 1.0 weight percent, based on the total weight of colorant and control agent, said dispersing aid being selected from the group consisting of rosin, hydrogenated rosin and esters of hydrogenated rosin.

7. A developer as in claim 6 in which the phthalo- 3,345,293 10/1967 Bartoszewicz 25262.1 cyanine pigment is phthalocyanine green. 3,349,703 10/ 1967 Varron et a1. 101-426 8. A developer as described in claim 6 wherein the carrier liquid is cycolhexane, GEORGE F. LESMES, Primary Examlner References Cited 5 M. B. WITTENBERG, Assistant Examiner UNITED STATES PATENTS US. Cl. X.R.

2,877,133 3/1959 Mayer 11737 961.2, 1.3; 11737(LX) 3,325,409 6/1967 Whitbread 252-62.1

"H050 UNITED STATES PATENT OFFICE 56g CERTIFICATE OF CORRECTION Patent No. 3 Dated ember 29, 1970 Inventor(s) Gene H- Robinson It is certified that error appears in the above-identified pater and that said Letters Patent are hereby corrected as shown below:

Column line hould read "1"; an:

Columri 6, line 57 after "flammability" ins ---control--.

Signed and sealed this 28th day of September 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Pat 

